I might get alot of rant for this but here goes - I would like to fight all alliances in war, big or small, no matter. So whatever you are planning to do with match making system, please don’t. remember how trophy system became fixed but turned into a tedious grinding process ? I am sure not many of us would like that to happen with wars too.
What I suggest is implementing a new game mechanic : morale.
Simply put, if a smaller alliance attacks a bigger alliance, it operates at, say, 150% efficiency (units). Same would go for defense. If a bigger alliance attacks a lower level alliance, it will operate below 100% efficiency.
Ofc the multipliers can be tweaked depending on alliance levels or trophy points.
The idea is to create an even playing field and encourage all alliances to participate and initiate strikes at whomever they choose to and not just alliances they are certain they can easily take down.